The Starks brought approximately 20,000 troops into the Riverlands (not their full strength, but all that could be raised at short notice) to aid their Tully allies. In theory, the Tullys and their vassals would amass at least another 20,000 troops, but early attacks and incursions by the Lannisters had reduced or scattered these forces. The precise Stark/Tully strength is difficult to calculate but on paper should have stood at 40,000. In practice, it appears that the rebels never fielded this many troops at once during the conflict.

Lannister strength appears to have consisted of two armies at the outset of hostilities, one of 20,000 commanded by Lord Tywin Lannister and another of 15,000 commanded by Jaime Lannister. An additional force of 10,000 troops was being raised at Oxcross by Ser Stafford Lannister as hostilities continued.

Although the Tyrells concentrated their efforts in the war on subduing Storm's End and helping win the Battle of the Blackwater, they did send a force of under 10,000 under Lord Randyll Tarly to help defend Duskendale from the rebel forces. This Tyrell army remained encamped at Duskendale whilst fighting continued and played no further part in the conflict.

After 'kidnap' of Tyrion Lannister by Catelyn Stark, Tywin invade the Riverlands in retribution. He assembled two armies, one designed to besiege and hopefully capture Riverrun and the other to hold the crossings over the Green Fork and deny the movements of Stark reinforcements into the Riverlands. Tywin assumed that, as in earlier conflicts, Lord Walder Frey would remain neutral, preventing the Starks from crossing at the Twins.

The conflict opened with the Battle of the Golden Tooth . A small holding force had been sent close to the border between the Riverlands and Westerlands by Edmure Tully to deny or slow the Lannister advance. However, it was insufficient to deal with the 15,000 troops under Jaime Lannister, and was defeated and routed from the field. Jaime advanced on Riverrun, encircled the castle and took Edmure Tully prisoner. Meanwhile Tywin led 20,000 troops to the ford over the Green Fork upon learning that the Stark host was advancing on the river.

Unbenknown to Tywin, Robb Stark had won the allegiance of Walder Frey by agreeing to marry one of his daughters. He had split his forces in two, leading his cavalry down the west side of the Green Fork towards Riverrun, whilst Lord Roose Bolton led the infantry down the east side towards the ford.

In the Battle of the Green Fork, the Stark host was denied a crossing of the river and was repulsed, although Roose Bolton was able to withdraw in good order. However, it wasn't until captured prisoners were interrogated that Tywin learned of Robb's plan, by which time it was too late to send reinforcements to Riverrun.

Robb's cavalry defeated Jaime Lannister's forces at the Battle of the Whispering Wood and took Jaime prisoner. At the Battle of the Camps the First Siege of Riverrun was lifted and most of the Lannister host in the west destroyed. Faced with the danger of being outflanked by Robb to the west and Bolton to the north, Tywin Lannister withdrew to Harrenhal and fortified it, aware that it could not be taken in battle easily. He also sent orders that a new Lannister host was to be assembled as soon as possible in the Westerlands and loosed a number of outrider and sellsword bands to put the Riverlands to the torch, burning out crops and destroying villages in an attempt to convince Robb to emerge from Riverrun and attack Harrenhal. This strategy was not effective.

Instead, Robb led an invasion of the Westerlands. Outflanking the Golden Tooth by an old goat track through the mountains, the Stark host fell on and destroyed the new Lannister host training at Oxcross. Aware that Casterly Rock and Lannisport were too heavily guarded to take easily, Robb instead undertook a campaign of destruction through the Westerlands, taking Ashemark and the Crag in short order.

With his commanders' anger mounting at the destruction of their homeland rising, Tywin led his forces from Harrenhal towards home. Unbeknown to Tywin, the whole situation was a trap, designed to dislodge his (temporarily reduced) forces from their impregnable fortress, bring them into the Westerlands, and then catch them in a three-pronged trap between Robb's forces in the west, Edmure's out of Riverrun and Roose Bolton's troops, who had come south and taken Harrenhal to deny the Lannisters a bolthole. Aware of the course of the conflict in the south, this would also draw the Lannisters far away from King's Landing when it was eventually attacked by Stannis Baratheon's forces.

The plan was not successful. Having not been informed of the plan, Edmure Tully assembled forces to deny the Lannisters a crossing of the Tumblestone, eventually defeating them and driving them back in disarray. Whilst this battle was a success, and one that was badly needed to win prestige for Edmure after most of the previous victories had been won by Robb Stark, it did disrupt Robb's plans. It also meant that once he heard of the alliance with the Tyrells arranged by Tyrion, Tywin was able to force-march due south to the Blackwater and reach King's Landing in time to help defeat Stannis' forces in the Battle of the Blackwater.

The new Lannister-Tyrell alliance proved bad news for Robb's forces, as it swung the balance of numbers decisively against him. In addition, news that House Greyjoy had entered the war by attacking the North had arrived, causing many of the Northern lords to demand that they return home to defend their own lands (ironically, the same tactic that Robb had deployed against Tywin). Finally, whilst convalescing at the Crag, Robb had taken Jeyne Westerling to wife, breaking his promise to the Freys, who had withdrawn from the alliance. With little choice, Robb knew he had to win back the Freys' trust and then drive the ironborn out of the North before being in a position to win the conflict in the south.

In the event this plan was unsuccessful. Following Robb's betrayal, Walder Frey offered Tywin Lannister an alliance instead in return for betraying and killing Robb Stark. Roose Bolton was brought into the plan as well, and arranged a disastrous attack on Duskendale that saw a third of the Stark foot destroyed by Gregor Clegane and Randyll Tarly. At the Twins the betrayal took place, the infamous Red Wedding which saw Robb and Catelyn Stark killed and Edmure Tully captured.

This effectively ended the war in this theatre, although Brynden Tully continued to hold Riverrun against a renewed Lannister siege. Jaime Lannister negotiated the castle's surrender, but Brynden escaped in the confusion.